The Chemical Makeup of the Universe

How does the universe produce and distribute elements?

The history of nucleosynthesis --- production of elements --- is recorded in the hot plasma of clusters of galaxies. Elements produced in stars and supernovae were gradually distributed and enriched this intergalactic plasma. The abundance patterns of various elements depends on the distribution of mass stars and the types of supernovae in the universe.

The knowledge of this abundance pattern is obtained through observation of stars, interstellar plasmas or supernova remnants in our Galaxy. With the XRISM high resolution X-ray spectrometer, we can detect rare elements that are thus far overlooked and measure their dissipation velocity.

As with rivers, seas, or atmosphere on the Earth, the hot plasma carries and circulates materials in the universe. The materials dissipate in interstellar space, and are recycled for stars and planets of the next generation or distributed into intergalactic space. With these flows, the circulation of elements is a dynamic process.

Distribution map of interstellar matter

Clues to the "missing baryon problem"

The dominant components of the hot plasma mass are protons and neutrons, two baryons that play important roles in the material history of the universe. However, these baryons were not always "hot". When the first baryons were produced in the young hot universe, they were fully ionized. As the universe expands and is getting cooler, those ionized atomic nuclei captured electrons to become neutral atoms in about 300,000 years. After 300,000,000 years, the first of generation stars were born, and their radiation heated and re-ionized the atoms to produce hot plasmas.

Although we observe this hot plasma in the nearby universe, we hardly observe the "warm" plasma during the age of re-ionization. Neutral or low temperature gas is observed with radio waves or optical light. We observe the hot plasma in X-ray band. However, the rest of the "warm" plasma ---- of 10,000 to a few 1,000,000 Kelvin is difficult to observe. These warm plasmas emit ultraviolet (UV) radiation. It is difficult to observe UV radiation from large distances, since UV radiation is easily absorbed by the large amount of hydrogen gas in the universe. We call those expected but not observed warm plasmas "missing baryons". The amount and element abundance of the missing baryons is an important piece to complete the puzzle of material history in the universe.

XRISM is able to observe the warm plasmas, although it cannot observe UV. With a prism, you may have observed dark lines in the spectrum of light from the Sun. These absorption lines are caused by the relatively low temperature chromosphere above the sun photosphere. With XRISM, we are able to observe the warm plasma through absorption features in the background X-ray source spectrum. We use distant but bright active galactic nuclei, for example, as the background light. The precise spectroscopy of XRISM is essential to determine the warm plasma's temperature and elemental abundance.